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A simple theoretical model that describes the pulsed Davies electron-nuclear double resonance (ENDOR) experiment for an electron spin S = ½ coupled to a nuclear spin I = ½ was developed to account for unusual W-band (95 GHz) ENDOR effects observed at low temperatures. This model takes into account the thermal polarization along with all internal relaxation processes in a four-level system represented by the electron- and nuclear-spin relaxation times T1e and T1n, respectively, and the cross-relaxation time, T1x. It is shown that under conditions of sufficiently high thermal spin polarization, nuclei can exhibit asymmetric ENDOR spectra in two cases: the first when tmix ≫ T1e and T1n, T1x ≫ T1e, where ENDOR signals from the α manifold are negative and those of the manifold positive, and the second when the cross- and/or nuclear-relaxation times are longer than the repetition time (tmix ≪ T1e ≪ tR and T1n, T1x > tR). In that case the polarization of the ENDOR signals becomes opposite to the previous case, the lines in the α manifolds are positive, and those of the manifold are negative. This case is more likely to be encountered experimentally because it does not require a very long mixing time and is a consequence of the saturation of the nuclear transitions. Using this model the experimental tmix and tR dependencies of the W-band 1H ENDOR amplitudes of [Cu(imidazole)4]Cl2 were reproduced and the values of T1e and T1x ≫ T1e were determined. The presence of asymmetry in the ENDOR spectrum is useful as it directly provides the sign of the hyperfine coupling. The presented model allows the experimentalist to adjust experimental parameters, such as tmix and tR, in order to optimize the desired appearance of the spectrum.
Department of Chemical Physics, Weizmann Institute of Science, Rehovot, 76100, Israel 2:
Facultät für Physik and Geowissenschaften, Universität Leipzig, Linnestrasse 5, Leipzig, D-04103, Germany